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v3.25.2

Note 1 - Basis of Presentation and Summary of Significant Accounting Policies (Details Textual) | Jun. 30, 2025
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Number of Bank Branches | 27

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- Definition Represents the number bank branches. + References No definition available. + Details
Name: | fsbw_NumberOfBankBranches
Namespace Prefix: | fsbw_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant